1. Field of the Invention
The present invention generally relates to out-of-bounds sensors and, more particularly, to optical tennis court boundary sensors.
2. Description of the Prior Art
Tennis is a recreational activity played on a rectangular tennis court with a net separating opposing players, or sides. The number of players on the tennis court varies depending on whether the game is singles or doubles tennis. In singles tennis, each side is composed of one player and, in doubles tennis, each side is composed of two players.
In the most basic description of the game, tennis players use tennis rackets to hit a tennis ball back and forth across the net. The tennis ball is hit until a player fails to return a tennis ball over the net and inside the tennis court boundaries, which are defined by various lines, including sidelines, baselines, service lines and a center line.
Tennis is a sport, among a number of sports, in which fans pay money to watch professional tennis players compete, and, consequently, cash awards are offered to the winning competitors. Amateur tennis, however, is an extremely popular game in its own right, played by millions of people throughout the world. In amateur tennis, the players typically keep score themselves and decide when a ball lands outside the boundaries of the tennis court. Hence, during service of the tennis ball, each player must monitor all service court lines. After service, only the sidelines and the baselines need be observed.
In contrast to amateur tennis, professional tennis matches, referred to as tournaments, require impartial officials to call balls as in or out-of-bounds. At such a tournament (one of the more famous tournaments is that hosted in Wimbledon, England every year), a chair umpire sits on a high chair overlooking the players and the court. The chair umpire functions as the ultimate authority over a squad of linesmen who watch the court lines and call balls. A full complement of linesmen includes, on each side of the court, a service line-linesman, a baseline-linesman, a center line-linesman, and two sideline-linesmen. Therefore, a complete squad of tennis officials comprises a chair umpire and ten linesmen. In addition, most tournaments will have a net umpire, and some tournaments will even have a foot fault judge. With so many officials monitoring lines for out-of-bounds balls, the tennis players on the court must indeed feel overwhelmed-
Amateur players, by contrast, having no impartial observers to watch the lines, must make out-of-bounds calls themselves. Such judgment calls are inherently imprecise, and may even be unfair. In general, the tennis player on the side of the court where the ball lands is best able to see if the ball is out-of-bounds. Thus, even if unintentional, players have an unconscious bias to call close balls as out-of-bounds. Moreover, players naturally tend to concentrate more on making contact with an in-flight tennis ball, rather than on watching where a passed ball lands.
Of course, in professional play, lack of concentration by a linesman may also lead to inaccurate calls. This conclusion becomes all the more evident when one considers the context of professional play which includes high speed imparted on tennis balls by professional players, and a variety of spins, bounces and skidding balls found in such hard-hitting games. It should be noted that at international tournaments the consequences of one bad call can be, not only the loss of a cash award, but also a lost opportunity for future revenue generated from promoting consumer products. Clearly then, amateur and professional players invariably would prefer consistent and precise calls provided by automated boundary sensors, over the caprices of human error.
Nonetheless, for a number of years now, precision out-of-bounds sensors have been a continuing holy grail. Many groups have tried to solve the out-of-bounds dilemma by either placing sensors adjacent to the tennis court boundary lines on the court, or immediately under the court. For example, pressure sensitive switches can be installed beneath tennis court boundary lines. The network of switches, upon detecting a downward force, cause a small current to drive a relay connected to an audiovisual signalling device. As another example, special semi-metallic balls can be manufactured to function in combination with electromagnetic line sensors such as conductive and magnetic stripes. Closely spaced leads are affected by contact with the special ball and cause an audiovisual device to signal an out-of-bounds condition.
The principal drawback of the pressure switch system is that individual sensors cannot be calibrated to discriminate between a bouncing ball and a player's foot. Alternatively, the inconvenience of using a special ball, in conjunction with the cost of permanently modifying a tennis court, are burdensome hurdles to leap for most potential users of the electromagnetic sensor systems.
More recently, optical out-of-bounds sensors have been disclosed which employ interruptions in light beams to detect out-of-bounds balls. The patent to Grant (U.S. Pat. No. 3,982,759), for instance, describes a linemonitoring apparatus which can be implemented using pressure switches or light interference detectors. According to the Grant patent, a timing circuit measures the length of time that laser light is interrupted, and if the interference is for longer than 50 milliseconds, the sensor assumes that the interference is due to a player. Oberan (U.S. Pat. No. 4,528,548) discloses another kind of optical out-of-bounds sensor for racquetball or handball. The Oberan patent is similar to Grant since it uses a timer to time the interruption of the light beam, and thereby discriminate between a player's foot and a tennis ball.
Other out-of-bounds systems have used more than one light beam to distinguish between a foot and a ball interruption. Chen, et al., (U.S. Pat. No. 4,004,805), shows a dual beam laser which, used in combination with a system of mirrors, completely surrounds the tennis court boundaries. The dual light beams include an inner, timed beam, inside the tennis court boundary line, and an outer, untimed beam, outside the tennis court boundary. The sensor discriminates the type of beam interruption by either detecting a long period of inner beam interference, and assuming the interference was made by a foot, or presuming a foot interference when both beams are interrupted for a short period, but almost simultaneously. Only when the outer beam is momentarily interrupted is an out-of-bounds indication made.
As yet another example of a dual light beam system, Wilson, et al., (U.S. Pat. No. 4,422,647), discloses an out-of-bounds detection system for a volleyball court. Wilson, et al., shows a top light beam, located parallel to a bottom light beam positioned over each court line. An edge-sensitive timer circuit measures the duration of a light beam interference; when the top beam is broken, the timer is reset and the bottom beam triggers on a negative edge from a HIGH voltage (not broken) to a LOW voltage (broken). Thus, when the top photo-transistor is HIGH (not broken), and the bottom light beam goes LOW (broken), the timer starts a count-down until either time expires or the timer is reset. If the top beam goes LOW (broken), then the timer is reset and an LED, indicating an out-of-bounds ball, goes off.
Upon close inspection of these prior technologies, serious shortcomings are evident. The pressure and electromagnetic sensors of the prior technologies require extensive permanent equipment to be embedded in the court, and sometimes such systems even require a special ball. Optical sensors have their own unique problems, and they also share some common problems with non-optical sensors. For example, in Grant, the fact that the tennis court boundary is protected by a single beam width means that hopping or skidding balls, as typically found on clay courts, are detected as foot interferences. The Wilson system has a similar problem; however, Wilson has an advantage that one beam is high enough above the ground to detect foot interruptions. The deficiency in Wilson seems to be that the timer requires exact simultaneous interruption of both beams for the system to not trigger an out-of-bounds condition.
A common shortcoming of all the current technology optical sensors is that tennis boundary coverage is not sustained when a foot interferes with a light beam, since the interruption by the foot, causing a long pulse, will overlap in time the short pulse caused by a ball.
Consequently, a need exists for improvements in tennis court out-of-bounds sensors which will result in an absence of extensive permanent tennis court equipment, a multi-beam coverage for each boundary line, an improved mechanism to distinguish player interferences from ball interferences, and a means to sustain tennis court coverage when players interfere with light beams.